What is this gas cap from?

Bought it at an estate sale today for $1. It was too cool to pass up, but I don't know what it fits. Any ideas?

 

reminds me of mid '50's rambler, sorry pumped a lot of gas in my time but I cant remember them all......

 

My dad had a boat with a very similar cap on it, it was oddly shaped so that it would sit flush on the top of the hull. Can't say for sure but that's what it looks like to me.

 

'60s something, and likely a station wagon. Most all else had their caps hidden.

Cosmo